Hi! I am having the same issue. I enrolled in the Apple Developer Program 24 hours ago.
I can access the Apple Developer Portal and the subscription is active, but I can not access App Store Connect. Error is ITC.signin.error.invalidUser
I raised multiple requests to Apple Developer Support and I have received responses promising to call me within the day. Received one call, robotic voice says to press 1 to accept the call and talk to an agent. Then silence for a few seconds and the call gets closed from their side... then I get an email saying they could not reach out to me and they will retry later. They did not retry (yet).
Will provide updates on my case too.
Honeyside
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Same issue here, on a newly opened enrolled account.